Wednesday, July 7, 2010

Taking a "brake" from the fuel system

Well, a "break," too. A break to start on the brakes, if you will. It's 90+ degrees outside with a humidity level to match, so I need to keep myself busy down in the shop. The fuel system fittings haven't arrived yet, so I thought I'd try to get started on the brake/rudder pedals. For those unfamiliar with the relationship between the two, here's a quick synopsis.

An aircraft is normally steered on the ground by use of the rudder pedals. The rudder pedals are attached via some linking mechanism to the nose wheel. Application of left rudder pedal will cause the aircraft to turn left, and vice-versa for the right. Braking of the aircraft is accomplished with differential braking. This means that the left or right brake can be applied individually, which enables the aircraft to turn about on its axis. In the case of the RV-12, there is no linkage between the rudder pedals and the nose wheel. All ground steering is therefore accomplished with differential braking until such time as sufficient airspeed is attained (typically during the takeoff roll) for the rudder to become effective. The differential braking is accomplished with "toe brakes." The rudder pedals either swivel at their base or have a separate pedal at the top that is activated by the top of the pilot's foot. In the case of the RV-12, each rudder pedal was a separate brake pedal.

The rudder pedals are a push type of control. You push on the right pedal for right rudder, and the left pedal for left rudder. Pushing on a pedal causes a tube to rotate. Flanges on the tube are connected to the rudder with steel cables. The upshot of all of this is that the tube needs something to support it yet allow it to rotate. This "something" is a set of plastic blocks. The first step is to cut them in half. The band saw had no trouble at all getting though them.

Because neither the band saw nor its operator is genetically capable of cutting a perfectly straight line, I marked each block half so that I'd be able to match them up again. Any deformities resulting from my poor directional skills with the saw would net out to a perfectly matched edge. Or so I like to think, anyway.

The two rudder pedal tubes are identical. Each has what the plans call a "short end" and a "long end." I set the two parts on top of each other with their short and long ends together.

Then I simply flipped one over to get the pair of them properly oriented to each other.

The two parts then get temporarily assembled into the plastic blocks. The plans show the need for washers to get the nuts to fit correctly on the bolts, but don't specify what size. I had a bag of the AN960 washers that eventually get used during the final installation lying around, so I just used those.

As it sits on the bench, the rudder pedal assembly is actually upside down. When it eventually gets installed into the airplane, it will get turned over and the blocks will be on the top.

To install the brake pedals, the tubes that support them are slipped through the rudder pedals. It is important to note here that the "pedals" are actually just horizontal steel tubes. The picture below shows two of the rudder pedals with the brake pedal supports inserted.

The brake pedals (which are also tubes) will pivot on the bare steel parts of the rudder pedal tubes. Trust me, it's all going to make sense soon. The brake pedals are going to be slid onto these tubes and match drilled for a bolt to hold them on. When a pedal is pushed, it will rotate the tube it is bolted to. That will move the flanges that you can see on the ends of the tubes, between the rudder pedals. The flanges will be attached to brake master cylinders. It's really quite clever.

To lock the pedals into place to aid in the drilling, one of the brake support tubes is inserted through the rudder pedals on the other side

It was a bit of a tight fit. The pedals didn't line up perfectly up & down, so it took a bit of twisting and turning to the brake support through both rudder pedals.

Back to the other side, two brake master cylinders need to be temporarily installed. I marked the 'L' and 'R', but these are parts that I don't think will fit in the wrong way if you lose track of which is which.

Here they are installed:

Now you can start to see how they're going to work. Once the actual brake pedals are bolted onto the bare steel part of the rotating tube, you can see how the rotation of the tube will activate the brake master cylinder.

The idea is for the brake pedals to be perpendicular to the rudder pedal assembly, so I used a square to get them as well lined up as I could.

It's hard to see what's what in that picture (the replacement photographer is much better about making sure my bald(ing) spot is out of the picture, but his overall composition skills are weak) so I added arrows to show the two parts of the brake pedal. Those are the two points that are used to ensure a close approximation of perpendicularity. I say "approximation" because 1) the rudder pedal tubes block good access for the square to align the inner brake pedal, and 2) because I had a great deal of trouble in keeping the pedals from moving as I clamped them in place. Oh, and 3) I also had trouble keeping them from moving as I was drilling them because my clamps suck. Hopefully the alignment isn't super critical.

The holes get drilled #30, then #12, then 1/4". There is not a single one of those sizes that wasn't an absolute bugger to drill. My little 12 volt lithium drill is wonderful for almost every situation, but drilling through hardened steel is not one of its strong suits. And quite painfully, there are eight holes to be drilled. With three separate sizes for each, it was a good two dozen holes to drill. I was mighty glad when it was done. At one point I got a 1/4" bit well and truly stuck, and I think I came very close to stripping the gears in the drill.

What with these being match drilled holes, the odds are enormous against them ever actually matching. That said, it's prudent to mark each part in order to aid in getting them back into position after removing them for deburring.

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